1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotography method or an electrostatic method, such as a copying machine or a printer, and more particularly to an image forming apparatus using a contact transferring method.
2. Related Background Art
Image forming apparatuses of a transfer type mainly use contact transferring means, especially such means of a roller transfer type to transfer a toner image formed and borne on an image bearing body onto transferring material. This is partly because, as contrasted with transferring means using a non-contact type corona charger, contact transferring means does not produce ozone. Contact transferring means has a transferring member which forms the image bearing body and a pressing nip transfers a toner image onto transferring material inserted into the pressing nip. Contact transferring means of a roller transfer type uses a roller (transferring roller) as a contact transferring member. Contact transferring means has advantages of stable transferring material conveyance etc.
Contact transferring means of a roller transfer type uses as a contact transferring member a transferring roller (electrically conductive elastic roller) with an intermediate-resistance elastic layer whose resistance is adjusted to 1xc3x97106 to 1xc3x971010 xcexa9. The roller is brought into contact with an image bearing body (hereinafter called a photosensitive drum) to form a pressing nip, or a transferring nip, between the photosensitive drum and transferring roller. By applying a transferring bias to the transferring roller while transferring material is conveyed, being nipped at the transferring nip, contact transferring means gives transferring material a charge opposite in polarity to a toner image to transfer the toner image from the photosensitive drum onto the transferring material.
The transferring roller has an elastic layer whose resistance is adjusted as appropriate, using rubber or sponge in which electrically conductive inorganic particles are dispersed, such as carbon particles are dispersed, or electrically conductive ion rubber into which surfactant or the like is kneaded. As is well known, the resistance of the transferring roller changes by one order or more of magnitude due to variations during production, changes in temperature and humidity, and prolonged periods of use.
To keep running an appropriate current through a transferring roller with such changing resistance, a transfer voltage can be applied to the transferring roller by a xe2x80x9cconstant-current applying method.xe2x80x9d In this case, however, transferring material is usually used whose width is smaller than the maximum width of paper which can pass through the apparatus. Thus when an area occurs which no paper passes, with the photosensitive drum and transferring roller in direct contact with each other in the direction of the length of the transferring nip, current concentrates on the area, so that current feed to the transferring material is insufficient, causing a poor transfer.
To run an appropriate current through the transferring roller irrespective of transferring material size, many image forming apparatuses use a xe2x80x9cconstant-voltage applying method.xe2x80x9d To run an appropriate current according to transferring roller resistance, varying with production conditions and the environment, the constant-voltage applying method exercises active transfer voltage control (ATVC) or programmable transfer voltage control (PTVC). Under ATVC method, a voltage which is produced when a constant current passed through the transferring roller during paper feed is applied to the roller before transfer is kept and applied during transfer. Under PTVC method, on the other hand, a voltage calculated by substituting into a predetermined control equation a voltage which is produced when a constant current is passed through the transferring roller before paper feed is applied during transfer. The constant-voltage applying method detects the impedance of a transferring system by ATVC method, PTVC method, or the like to apply a transfer voltage that allows a current within an appropriate range to run.
A bias can more accurately be controlled by PTVC method than by ATVC method, whose system, consisting of hardware configuration circuits, can provide only a few values of bias to be applied. Moreover, PTVC method is economically advantageous because it needs no hardware circuits for voltage control.
PTVC method will be described in more detail below. When no paper is fed before printing, the level of a pulse width modulation (PWM) signal is increased stepwise toward a target current value, with the photosensitive drum surface charged, to apply a voltage to the transferring roller. A voltage at which the target current value is reached is held as Vt0. A transfer voltage Vt during printing, suitable for Vt0 is determined from Vt0 and a transfer output table stored in advance in a control circuit CPU. A PWM signal corresponding to the transfer voltage Vt is output to apply the voltage Vt to the transferring roller during printing.
Determining the transfer voltage Vt during printing with reference to the voltage Vt0 produced by the transferring roller at a constant current allows the best voltage to be applied during printing according to the resistance of the transferring roller, thus providing a good image, using transferring rollers widely ranging in resistance.
However, the conventional transfer voltage control methods pose the following problems.
Conventional transferring voltage control methods, such as ATVC and PTVC, determine transfer voltage to be applied during transferring from a voltage which is produced when a constant current is passed through the transferring roller, with no transferring material at a transferring nip. In this way, the methods are adapted so that if transferring roller resistance changes, a suitable transferring bias can be applied accordingly by detecting the impedance of the entire transferring system when no paper is fed.
However, if transferring material resistance is too high or low, that is, transferring material whose resistance differs widely from an estimated value is used, a transfer current to be run during printing may be outside the suitable transfer current range.
If transferring material impedance significantly changes, transfer current may be excessive or insufficient, thus resulting in a poor image. Because more image forming apparatuses have recently been used worldwide, types of transferring material used for printing are increasing. With this trend, transferring material with a variety of resistances are used, so that it is difficult to form good images irrespective of the environment and types of transferring material.
It is possible to have a user specify the type of transferring material in a special paper mode to optimize transfer voltage control. However, doing so is not preferable because specifying the type troubles the user.
Japanese Patent Application Laid-Open No. 4-251276 proposes that faulty transfer due to a change in transferring material resistance is prevented by determining transfer voltage from a current which runs when a voltage is applied to the transferring roller at the tip end of transferring material.
However, because this method incorporates the resistance of a combination of the transferring roller and transferring material, it cannot cover a change in either transferring roller resistance or transferring material resistance to be exact.
It is an object of the present invention to provide an image forming apparatus which does not cause a poor transfer in spite of environmental variations.
It is another object of the present invention to provide an image forming apparatus which can correctly transfer an image irrespective of the type of transferring material.
It is still another object of the present invention to provide an image forming apparatus which can cover an independent change in either transferring member resistance or transferring material resistance.
It is a further object of the present invention to provide an image forming apparatus comprising:
an image bearing body for bearing a toner image;
a transferring member for forming a nip with said image bearing body to pinch a transferring material and for transferring the toner image on said image bearing body to the transferring material;
voltage applying means for applying a transfer voltage to said transferring member;
voltage detecting means for detecting an applied voltage at which a predetermined current flows through said transferring member before a transferring operation;
current detecting means for detecting a current flowing when a voltage based on a detected voltage of said voltage detecting means is applied to said transferring member in a condition where a tip end of the transferring material is present at the nip; and
voltage determining means for determining the transfer voltage after the tip end of the transferring material, based on a detected current of said current detecting means.
Other objects of the present invention will be apparent from the following descriptions.